Apparatus and process for preparing semiconductor rods



R. J. BINDER May 23, 1967 APPARATUS AND PROCESS FOR PREPARINGSEMICONDUCTOR RODS Filed Oct. 13, 1964 2 Sheets-Sheet 1 RECTIFIER HIGHFREQUENCY GENERATOR ATTORNEY May 23,

1967 R. J. BINDER 3,321,299

APPARATUS AND PROCESS FOR PREPARING SEMICONDUCTOR RODS Filed Oct. 15,1964 2 Sheets-Sheet 2 I02 6o I 1o 62 I00 63 65 V RECTIFIER 74 64 72 4--.0- o 66 69 7e 7e I03 I I l v IoI 1 9a /99 W m 93 92 7| 67 I as 75 77 9|I CATHODE l/ FOLLOWER RECTIFIER T 79 so 9o I a MAGNETIC AMPLIFIER I x 2as 96 94 50 HIGH FREQUENCY 53 4 57 GENERATOR? 55 I I 82 i-E- FIG,2

' INVE'NTOR RICHARD J. BINDER EMA 041M ATTORNEY United States Patent3,321,299 APPARATUS AND PROCESS FOR PREPARING SEMKCONDUCTOR RODS RichardI. Binder, St. Louis, Mo., assignor to Monsanto Company, St. Louis, Mo,a corporation of Delaware Filed Get. 13, 1964, Ser. No. 403,490 15Claims. ((11. 75-1tb) This invention relates to the preparation ofsemiconductor rods and is particularly concerned with an apparatus andprocess for automatic thickness regulation incident to zone melting avertically positioned rod of semiconductor material.

Semiconductor materials are zone melted for a number of purposes. Forexample, zone melting operations are conducted to effect purification ofsemiconductor materials, to effect uniform distribution of carrierimpurities, in semiconductor materials, and to effect the transformationof polycrystalline semiconductor materials into a rod of semiconductormaterial formed from a single crystal. In other instances the zonemelting operation is conducted primarily to effect a change in thediameter of a rod of semiconductor material or to remove irregularitiesin the diameter of a rod of material. Instances where it is desirable toeffect a change in the diameter of a rod of semiconductor materialinclude the production of slim rods for use in a decomposer and, in manyinstances, the production of seeds for use in the manufacture of singlecrystal material.

In all operations of the above type it is normally desirable that therod of semiconductor material resulting from the zone melting operationbe predominantly of a uniform diameter. For example, in the manufactureof mono-crystalline semiconductor rods it is of prime importance thatthe semiconductor rod produced be of controlled uniform diameter. Byaccurately controlling the diameter such advantages accrue to theproduct as exhibiting a more uniform radial resistivity gradient andimproved yields due to a more uniform diameter as well as the ability tomake larger diameter rods with improved yields.

Various automatic methods for controlling the diameter of thesemiconductor rod being formed during the Zone melting process are knownand such methods rely on monitoring the diameter by optical means,temperature-sensitive means, electrical means and various combinationsof these means. For example, in the automatic control by electricalmonitoring means the DC. anode current driving the high frequency sourceconnected to the inductive heating element (work coil), which changeswith a changing thickness of the rod, is monitored and used to controlthrough manipulative arrangements the diameter of the semiconductor rod.Such control means has many limitations among which are its sensitivityand responsiveness to changing conditions.

As will be more fully discussed hereinafter, this inven tion is directedto an improved electrical control means which is highly versatile inthat it can be used with many and various types of mechanicalarrangements for controlling the diameter of semiconductor rods andwhich minimizes or obviates the limitations of the foregoing methods.

According to the present invention, there is provided a process andapparatus for automatic thickness regulation of a semiconductor rod asit is being formed from a molten zone of semiconductor materialutilizing the voltage of the high frequency work coil used to producethe molten zone by inductive heating, which voltage changes with achanging thickness of the semiconductor rod, for controlling means toeffect a change in the diameter of the interface of resolidificationwhich restores the voltage to a predetermined voltage when said voltagechanges therefrom, all of which will be more fully discussedhereinafter. As is known, the interface of resolidification, that is,the boundary between the molten zone and the solid semiconductor rod ofdesired thickness formed therefrom, when varying in diameter results ina variation of the load impedance. This load variation has now beenfound to effect a proportional variation in the work coil voltage suchthat a definite voltage value will correspond to a definite diameter ofthe semiconductor rod.

The various objects and features of the invention will be brought out inthe description and in order to facilitate the description andunderstanding of the invention, reference is made to the appendeddrawings in which:

FIGURE 1 shows the principles underlying the operation of one embodimentof the present invention; and

FIGURE 2 shows the principles underlying the operation of anotherembodiment of the present invention.

Referring now to FIGURE 1 which illustrates schematically one embodimentof the present invention, a vertically positioned semiconductor rod 1,such as a polycrystalline silicon or germanium rod, is end supported at2 and 3 as shown. A high frequency work coil 4 is mounted concentricallyabout a portion of the rod and when energized establishes a molten zoneWithin the portion of the rod. The foregoing arrangement may besurrounded by a tube (not shown), such as a quartz tube, through which aprotective atmosphere, such as argon or hydrogen, is passed or the tubemay be provided with a vacuum. The work coil is arranged to move axiallywith respect to the rod at a given speed while being energized by a highfrequency generator 5. Within the circuit of the work coil and the highfrequency generator a resonating capacitor 6 is provided in parallel topermit a large circulating current in the resonant circuit of which thework coil is a part, while the high frequency source has to supply onlya relatively small current. The frequency of the generator 5 ispreferably so adjusted that it is almost, but not quite, identical withthe natural frequency of the resonant circuit so that the generatoroperates on a flank of the resonance peak in thecurrent-versus-frequency characteristic of the resonant circuit. Inoperation, the work coil when energized moves axially of the rod in thedirection indicated by the arrow 7 and establishes a moving molten zonewithin the rod which by controlling the conditions thereof permits asemiconductor rod of desired diameter to be produced. In. cases such asthe production of a purified monocrystalline semiconductor rod, theoperation can be performed in a reiterative manner in order to obtain adesired degree of purity.

The means for controlling the diameter of the finished semiconductor rodis provided by an electrical circuit, electrically connected in parallelwith the high frequency power circuit, which monitors the voltage of thepower circuit and, when the voltage of such circuit starts to changefrom a predetermined value, the change is immediately detected by themonitor circuit which activates and controls a mechanical arrangementfor compressing or stretching the rod in order to effect a change in thediameter of the interface of resolidification which restores the voltageto the predetermined value thereby producing a semiconductor rod of adesired uniform diameter.

The monitor circuit is provided with measuring capacitors, 20 and 21,which prevent any appreciable current from being drawn from the highfrequency power circuit, a rectifier 22 which rectifies the alternatingcurrent voltage to a pulsed direct curpent voltage and which may beprovided with a smoothing capacitor, if desired, and connected J inseries therewith, a cathode follower 23 which permits the pulsed directcurrent voltage from the circuit to be used and/or measured withoutdrawing any appreciable current from the circuit. A resistor 24 isprovided across the pulsed direct current voltage and connected inseries therewith a polarized relay 25 and an input resistor 26 providedwith a set point input. Connected in parallel with the input resistor 26is a battery 27. The polarized relay 25, through the contact spring 28,positions, by way of a motor 29 and a suitable gear (not shown), atoothed wheel 30 which drives a rack arrangement 31 attached to one endof the semiconductor rod and which can eflect, if desired, either acompressing or stretching force on the rod.

In the operation of the electrical monitor means, the input resistor 24is adjusted by the set point input so that there is no voltage dropacross the polarized relay 25 when the work coil is operating at itspredetermined voltage value corresponding to the desired thickness ofthe rod and the contact spring 28 of the polarized relay 25 ispositioned in a neutral position as shown. If the diameter of theinterface of resolidification changes thereby changing the thickness ofthe rod during the operation, the voltage supplied by the high frequencypower source to the work coil will also change due to a change in theelectromagnetic coupling between the semiconductor rod and the workcoil, and this change in voltage will immediately be detected by thecapacitively coupled monitor circuit which will register a change involtage across the polarized relay 25 without materially influencing thevoltage of the work coil, thereby establishing a current which willcause the relay 25 to energize and position the contact spring 28 toestablish contact with either point 32 or 33 depending on the voltagechange taking place. When closure of the circuit is established bycontact through the point to the motor 29, the motor will operate thetoothed gear 30 which effects movement of the rack 31 such that acompressing or stretching force is maintained on the rod until thesemiconductor rod is again at the desired uniform thickness and thevoltage in the work coil circuit is again at its predetermined value.For example, if the diameter of the semiconductor rod starts to becomesmaller during the operation, the voltage of the work coil will start toincrease and this increase in voltage will be detected by the monitorcircuit which will energize the relay 25 and position the contact spring28 in such a manner that it closes, for example, contact 33 to cause theoperation of the motor 29, so as to rotate the toothed wheel 30 in thedirection of the arrow 34, resulting in the compression of thesemiconductor rod until the work coil voltage is again at itspredetermined voltage value, causing the relay via the monitor circuitto restore the contact spring to the neutral position. In this manner,the thickness of the semiconductor rod is controlled by the highlyaccurate and rapidly reacting voltage change in the monitor circuitwhich occurs when the diameter of the interface of resolidificationchanges thereby permitting a semiconductor rod to be produced with adesired uniform diameter.

With particular reference to FIGURE 2 of the drawings there isillustrated schematically another embodiment of the present invention.In this figure there is shown a lower rod holding means 50 designed tosecure the lower end of a rod of semiconductor material 52 in a verticalposition. The rod holder 50 is supported upon a threaded verticallydisposed shaft 53 which engages an internally threaded collar 54carrying a horizontally disposed gear 55, the collar 54 being rotatablysupported upon a support frame member 56 bored to receive the verticalshaft 53. Horizontally disposed gear 55 is in operative engagement witha second gear 57 which is driven through a conventional reduction geartrain, not illustrated, by a suitable electric motor 58. Shaft 53 isfree to move vertically but is keyed against rotation so that byrotation of gear 55, the shaft 53 and rod holder 50 can be movedvertically as desired.

The reference numeral 60 generally indicates a second rod holding meanswhich in this instance is shown to comprise a pair of grooved rollers 61and 62. The roller 61 is mounted upon a shaft 63 which is journaledthrough.

a pair of support arms, one of which is shown at 64, on either side ofthe roller 61. Roller 62 is similarly mounted upon a shaft 65 which isjournaled at either end through a pair of support arms one of which isshown at 66. Support arms 64 and 66 are respectively carried by a pairof shafts 67 and 68 which are journaled through a support member 69. Thesupport arms 64 and 66 are rotatable about shafts 67'and 68,respectively, but relative movement between the arms and shafts 67 and68 is preferably not friction free for reasons which will subsequentlybe made apparent.

Secured to one end of roller 61 is a gear 70 which meshes with a gear 71secured to shaft 67. Similarly, secured to one end of roller 62 is agear 72 which meshes with a gear 73 secured to shaft 68 and which inturn meshes with gear 71 so that gear 71 drives both rollers 61 and 62but in opposite directions. A suitable electric motor schematicallyillustrated at 74 is provided for driving shaft 67, to which gear 71 issecured, through a suitable conventional gear reduction train, notillustrated.

The reference numeral 75 designates a rotary support platform upon whichthe rod holding means 60 is positioned. While it is not necessary thatplatform 75 rotate it has been found and is well known in the art thatrotation of one or both rod holding means securing the ends of a rod ofsemiconductor material being zone refined is normally desirable andgives improved results. A small pulley 76 driven by an electric motorschematically illustrated at 77 is provided for rotating platform 75 bymeans of a belt 78 running in a suitable groove in the periphery of theplatform. To reduce vibration and assist in maintaining the upper solidportion of rod 52 in proper alignment, a pair of freely rotatable idlerrollers 79 and 80 are disposed below platform 75 and are yieldablybiased toward each other to form a nip through which rod 52 passes.

The reference numeral 81 designates a conventional high frequency workcoil (heater coil) operatively disposed around the rod of semiconductormaterial 52. The work coil 81 is electrically connected to a suitablesource of high frequency current such as a conventional high frequencygenerator schematically illustrated at 82 so that by passing a highfrequency current through the coil 81 a molten zone can be created inthe upstanding rod of semiconductor material 52. Within the .circuit ofthe Work coil and the high frequency generator a resonating capacitor 83is provided in parallel to permit a large circulating current in theresonant circuit of which the work coil is a part, while the highfrequency source has to supply only a relatively small current. Thefrequency of the generator 82 is preferably so adjusted that it isalmost, but not quite, identical with the natural frequency of theresonant circuit so that the generator operates on a flank of theresonance peak in the current-versusfrequency characteristic of theresonant circuit.

With the upper extremity of the rod held by rollers 61 and 62 in contactwith lower extremity of the rod held by holder 50 through the moltenzone, the motors 58, 77 and 74 are placed in operation and the rate ofoperation of motors 58 and 74 are correlated so that the upward movementof the lower solid extremity of rod 52 relative to the rate of upwardmovement of the upper solid extremity of rod 52 is such that a smalldiameter rod of the desired mean diameter is withdrawn from the moltenzone. As the lower extremity of rod 52 is pushed upwardly into themolten zone by means of motor 58, the upper solid extremity of rod 52 iswithdrawn from the molten zone by motor 74 at a rate such that the twosolid portions of the rod are continuously receding from each other, andthe large diameter feed stock is transformed into a small diameterproduct by means of the zone refining operation.

It was previously mentioned that movement of arms 64 and 66 relative toshafts 67 and 68 is preferably not friction free. It will now be seenthat the reason for this is that, in normal operation, friction betweenshaft 67 and arm 64 and between shaft 68 and arm 66 results in rollers61 and 62 being urged toward each other so that the upper solid portionof rod 52 is gripped more forcefully. This reduces the possibility ofslippage between rod 52 and rollers 61 and 62 and reduces the extent towhich rollers 61 and 62 need be spring biased toward each other.

The means for controlling the diameter of the finished semiconductor rodis provided by an electrical circuit, in parallel with the highfrequency power circuit, which monitors the voltage of the power circuitand, when the voltage of such circuit starts to change from apredetermined value, the change is immediately detected by the monitorcircuit which activates and controls means for restoring the voltage tothe predetermined value thereby maintaining the interface of[resolidification with a uniform desired diameter and thus producing asemiconductor rod of a desired uniform diameter.

The monitor circuit is provided with measuring capacitors, 90 and 91,which prevent any appreciable cunrent from being drawn from the highfrequency power circuit, a rectifier 92 which rectifies the alternatingcurrent voltage to a pulsed direct cunrent voltage and which may beprovided with a smoothing capacitor, if desired, and connected in seriestherewith a cathode follower 93 which draws no appreciable current fromthe work coil circuit and which provides a biasing means for controllinga magnetic amplifier 94. The amplifier, connected in series with thecathode follower, amplifies the pulsed direct current voltage input. Theamplifier, in turn, is connected through suitable leads 95 and 96 to asource of alternating current electricity. A resistor, indicated by thereference numeral 97, is disposed in lead 95 between the source ofelectric power and the magnetic amplifier, and a lead 98 is connected tolead 95 intermediate resistor 97 and magnetic amplifier 94. It will beseen that with this arrangement the voltage across leads 98 and 99varies with changes in the voltage across the work coil circuit withoutmaterially influencing the work coil voltage. A lead 99, connected tolead 96, together with lead 98 supplies power to an adjustableauto-transformer 100 which in turn is connected to a full wave rectifierindicated by the reference numeral 161. Rectifier 101 supplies acontrolled rectified voltage to motor 74 through commutator rings, notillustrated, and leads 102 and 103. The voltage supplied to motor 74with this arrangement can be manually controlled by means of variabletransformer and is automatically varied in response to variations in thevoltage of the work coil circuit.

In the operation of the electrical monitor means, the motor 74 isadjusted to a proper pull rate, i.e., the rate at which the upper solidextremity of rod is withdrawn from the molten zone, when the work coilis operating at its predetermined voltage value corresponding to thedesired thickness of the rod. During the pulling operation, if thediameter of the interface of resolidification changes thereby changingthe desired thickness of the rod, the voltage supplied by the highfrequency power source to the work coil will also change due to a changein the electromagnetic coupling between the semiconductor rod and thework coil, and this change in voltage will immediately be detected bythe capacitively coupled monitor circuit which will register a change involtage across the magnetic amplifier 94, i.e. leads 95 and 96, which inturn increases or decreases the speed of motor as the case may be. Themotor speed, therefore, will adjust the pull rate such that the diameterof the interface of resolidification returns to its desired dimension,thereby, restoring the voltage in the work coil circuit to itspredetermined value, thus maintaining the semiconductor rod at the desired uniform thickness.

It will be understood that rod diameter control apparatus in accordancewith this invention can be utilized with any zone refining apparatus,whether of the gas or vacuum type. It will likewise be understood thatvarious modifications may be made in the electrical or mechanical sytemsof the embodiments illustrated without departing from the spirit of theinvention.

Having thus described my invention and several specific embodimentsthereof, what I desire to claim and secure by Letters Patent is:

1. In a method of zone melting a rod of semiconductor material in whichthe rod is vertically supported at both ends and a molten zone, producedby inductive heating through the medium of a high frequency work coilsurrounding said rod, is formed in said rod intermediate the two endsthereof, said molten zone being caused to move relative to said rodalong the longitudinal axis thereof to establish an interface ofresolidification between said molten zone and the rod formed therefrom,the improve ment which comprises utilizing the voltage of said workcoil, which voltage changes with a change in the diameter of theinterface of resolidification, for controlling means to effect a changein the diameter of the interface of resolidification which restores saidvoltage to a predetermined voltage when said voltage changes therefromthereby maintaining said interface of resolidification with asubstantially uniform diameter and thus forming said semiconductor rodwith a substantially uniform diameter.

2. In a method of zone melting a rod of semiconductor material in whichthe rod is vertically supported at both ends and a molten zone, producedby inductive heating through the medium of a high frequency work coilsurrounding said rod, is formed in said rod intermediate the two endsthereof, said molten zone being caused to move relative to said rodalong the longitudinal axis thereof to establish an interface ofresolidification between said molten zone and the rod formed therefrom,the improvement which comprises monitoring, by means of a controlcircuit electrically connected in parallel to said work coil, thevoltage of said work coil, which voltage changes with a change in thediameter of the interface of resolidification, and activating means toeffect a change in the diameter of the interface of resolidificationwhich restores said voltage to a predetermined voltage when said voltagechanges therefrom thereby maintaining said interface of resolidificationwith a substantially uniform diameter and thus forming saidsemiconductor rod with a substantially uniform diameter.

3. A method according to claim 2, wherein said control circuit iscapacitively coupled to said work coil.

4. A method according to claim 3, wherein said means eifects a change inthe diameter of the interface of resolidification by an applicablestretching and compressing force applied to said semiconductor rod, saidapplicable force being in the direction to restore the diameter of theinterface of resolidification to a substantially uniform diameter.

5. A method according to claim 4, wherein said force is applied to theend of said semiconductor rod toward which the molten zone is caused tomove.

6. A method according to claim 5, wherein said control circuit activatessaid means to reduce the diameter of the interface of resolidificationwhen the voltage of said work coil increases above the predeterminedvoltage.

7. A method according to claim 5, wherein said control circuit activatessaid means to increase the diameter of the interface of resolidificationwhen the voltage of said work coil decreases below the predeterminedvoltage.

8. In an apparatus for zone melting a rod of semiconductor materialcomprising upper and lower holding means for supporting the ends of arod of semiconductor material such that the longitudinal axis of saidrod extends vertically, a high frequency work coil surrounding said rodfor providing a molten zone by inductive heating in said verticallyextending rod of semiconductor material, traverse means for effectingrelative movement of said work coil longitudinally of said verticallyextending rod of semiconductor material to establish an interface ofresolidification between said molten zone and the rod formed therefrom,the improvement which comprises circuit means for monitoring the voltageof said work coil, which voltage changes with a change in the diameterof the interface of resolidification, and control means responsive tosaid circuit means to effect a change in the diameter of the interfaceof resolidification which restores said voltage to a predeterminedvoltage when said voltage changes therefrom to thereby maintain saidinterface of resolidification with a substantially uniform diameter thusforming said semiconductor rod with a substantially uniform diameter.

9. Apparatus according to claim 8, wherein said circuit means iscapacitively coupled to said work coil.

10. Apparatus according to claim 9, wherein said control means effects achange in the diameter of the interface of resolidification by anapplicable stretching and compressing force applied to saidsemiconductor rod, said applicable force being in the direction torestore the diameter of the interface of resolidification to asubstantially uniform diameter.

11. Apparatus according to claim 10, wherein said force is applied tothe end of said semiconductor rod toward which the molten zone is causedto move.

12. Apparatus according to claim 11, wherein said circuit meansactivates said control means to reduce the diameter of the interface ofresolidification when the voltage of said work coil increases above thepredetermined voltage.

13. Apparatus according to claim 11, wherein said circuit meansactivates said control means to increase the diameter of the interfaceof resolidification when the voltage of said work coil decreases belowthe predetermined voltage.

14. In an apparatus for zone melting a rod of semiconductor materialcomprising upper and lower holding means for supporting the ends of arod of semiconductor material such that the longitudinal axis of saidrod extends vertically, a high frequency work coil surrounding said rodfor providing a molten zone by inductive heating in said verticallyextending rod of semiconductor material, traverse means for effectingmovement of said work coil longitudinally of said vertically extendingrod of semiconductor material to establish an interface ofresolidification between said molten zone and the rod formed therefrom,the improvement which comprises a monitoring circuit comprised of, incombination, meas- '8 uring capacitors for preventing any appreciablecurrent from being drawn from said work coil, a rectifier for rectifyingthe alternating current voltage to a pulsed direct current voltage and acathode follower for premitting the pulsed direct current voltage to beused without materially influencing said work coil voltage, and controlmeans responsive to said circuit means comprised of, in

combination, a polarized relay disposed between said cathode followerand a direct current voltage source, a motor actuated by said polarizedrelay when a voltage drop is impressed across said relay, a toothedwheel driven by said motor when actuated and a rack arrangement attachedto the end of said rod for effecting a compressing and stretching forceon said rod when driven by said wheel.

15. In an apparatus for zone melting a rod of semiconductor materialcomprising upper and lower holding means for supporting the ends of arod of semiconductor material such that the longitudinal axis of saidrod extends vertically, a high frequency work coil surrounding said rodfor providing a molten zone by inductive heating in said verticallyextending rod of semiconductor material, traverse means for effectingmovement of said vertically extending rod through said work coil toestablish an interface of resolidification between said molten zone andthe rod formed therefrom, said traverse means adapted to withdraw saidrod from sail molten zone at a rate in excess of that at which said rodis passed to said molten zone, the improvement which comprises amonitoring circuit comprised of, in combination, measuring capacitorsfor preventing any appreciable current from being drawn from said workcoil, a rectifier for rectifying the alternating current voltage to apulsed direct current voltage, a cathode follower for permitting thepulsed direct current voltage to be used without materially influencingsaid work coil voltage and an amplifier for amplifying the directcurrent voltage, and control means responsive to said circuit meanscomprised of, in combination, an alternating current source, anadjustable auto-transformer and a rectifier for supplying a controlledrectified voltage to said traverse means.

References Cited by the Examiner UNITED STATES PATENTS 2,913,561 11/1959Rummel et a1. 219-10.43

3,046,379 7/1962 Keller et al 21910.43 X

3,265,470 8/1966 Keller 23-273 X HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

H. F. SAITO, Assistant Examiner.

1. IN A METHOD OF ZONE MELTING A ROD OF SEMICONDUCTOR MATERIAL IN WHICHTHE ROD IS VERTICALLY SUPPORTED AT BOTH ENDS AND A MOLTEN ZONE, PRODUCEDBY INDUCTIVE HEATING THROUGH THE MEDIUM OF A HIGH FREQUENCY WORK COILSURROUNDING SAID ROD, IS FORMED IN SAID ROD INTERMEDIATE THE TWO ENDSTHEREOF, SAID MOLTEN ZONE BEING CAUSED TO MOVE RELATIVE TO SAID RODALONG THE LONGITUDINAL AXIS THEREOF TO ESTABLISH AN INTERFACE OFRESOLIDIFICATION BETWEEN SAID MOLTEN ZONES AND THE ROD FORMED THEREFROM,THE IMPROVEMENT WHICH COMPRISES UTILIZING THE VOLTAGE OF SAID WORK COIL,WHICH VOLTAGE CHANGES WITH A CHANGE IN THE DIAMETER OF THE INTERFACE OFRESOLIDIFICATION, FOR CONTROLLING MEANS TO EFFECT A CHANGE IN THEDIAMETER OF THE INTER-